In recent years, with the request for speeding-up of signal transmission in electronic parts and electronic equipments containing such a part therein, there has been a demand for development of those low in electric resistance of wiring between electrodes as circuit boards for fabricating LSI packages such as BGA and CSP or circuit boards, on which these semiconductor devices are mounted. Therefore, it is extremely important to measure the electric resistance of wiring between electrodes of such a circuit board with high precision in electrical inspection of the circuit board.
In the measurement of an electric resistance of a circuit board, there has heretofore been adopted, for example, the four probe method, in which probes PA and PD for current supply and probes PB and PC for voltage measurement are pressed against and brought into contact with respective 2 electrodes 91 and 92 to be inspected, which have been electrically connected to each other in a circuit board 90 to be inspected, as illustrated in FIG. 11, a current is supplied between the probes PA and PD for current supply from a power supply device 93 in this state, and a voltage signal detected by the probes PB and PC for voltage measurement at this time is processed in an electric signal processor 94, thereby finding an electric resistance value between the electrodes 91 and 92 to be inspected.
In the method described above, however, the surfaces of the electrodes 91 and 92 to be inspected may be damaged by pressing the probes because it is necessary to bring the probes PA and PD for current supply and the probes PB and PC for voltage measurement into contact with the respective electrodes 91 and 92 to be inspected by considerably high pressing force, the probes are made of a metal, and the tips thereof are pointed. Therefore the circuit board becomes impossible to be used. Under such circumstances, the electric resistance measurement cannot be performed on all circuit boards to be provided as products, and so so-called sampling inspection has to be performed. After all, the yield of the products cannot be increased.
In order to solve such a problem, there have heretofore been proposed electric resistance-measuring apparatus, in which a connecting member coming into contact with electrodes to be inspected is formed by a conductive elastomer.
For example, (i) Japanese Patent Application Laid-Open No. 26446/1997 discloses an electric resistance-measuring apparatus, in which elastic connecting members formed of conductive rubber obtained by binding conductive particles with an elastomer are arranged at electrodes for current supply and electrodes for voltage measurement, respectively, (ii) Japanese Patent Application Laid-Open No. 2000-74965 discloses an electric resistance-measuring apparatus having a common elastic connecting member formed of an anisotropically conductive elastomer, which is provided so as to come into contact with surfaces of both electrode for current supply and electrode for voltage measurement electrically connected to the same electrode to be inspected, and (iii) Japanese Patent Application Laid-Open No. 2000-241485 discloses an electric resistance-measuring apparatus comprising a circuit board for inspection, on the surface of which a plurality of inspection electrodes have been formed, and an elastic connecting member formed of a conductive elastomer, which is and provided on the surface of the circuit board for inspection, wherein 2 electrodes of the inspection electrodes are selected in a state that electrodes to be inspected have been electrically connected to the plurality of the inspection electrodes through the connecting member, one of both electrodes is used as an electrode for current supply, and the other is used as an electrode for voltage measurement to measure an electric resistance.
According to such electric resistance-measuring apparatus, an electrode for current supply and an electrode for voltage measurement are brought into contact with electrodes to be inspected of a circuit board to be inspected through the elastic connecting member, whereby electrical connection is achieved, so that the measurement of the electric resistance can be conducted without damaging the electrodes to be inspected.
However, the measurement of an electric resistance between electrodes by the electric resistance-measuring apparatus of the construction (i) or (ii) involves the following problems.
In recent years, the size and pitch or interelectrode distance of electrodes in a circuit board have tended to become small for the purpose of achieving a high degree of integration. In the electric resistance-measuring apparatus of the construction (i) or (ii), however, it is necessary to electrically connect both electrodes for current supply and electrodes for voltage measurement at the same time to electrodes to be inspected in a circuit board to be inspected, which is subjected to electric resistance measurement, through the elastic connecting member, respectively. In the electric resistance-measurement apparatus for measuring an electric resistance as to a circuit board to be inspected, on which small-sized electrodes to be inspected have been arranged at a high density, accordingly, it is thus necessary to form electrodes for current supply and electrodes for voltage measurement in a state separated from one another, corresponding to individual small-sized electrodes to be inspected, in a region of an area equal to or smaller than a region occupied by the electrodes to be inspected, i.e., to form electrodes for current supply and electrodes for voltage measurement smaller in size than the electrodes to be inspected in a state separated from one another at an extremely short distance.
On the other hand, in order to improve productivity, a production process to produce a plurality of circuit board separated from each other by producing a combined circuit board composed of a plurality of circuit boards linked together from a base plate material, collectively conducting electrical inspection as to the respective circuit boards in the combined circuit board in this state and then cutting the combined circuit board is adapted.
However, since the area of the combined circuit board, which is an inspection target, is considerably large, and number of electrodes to be inspected is also extremely great, and, when multi-layer circuit boards are produced in particular, the number of steps in the production process thereof is great, and the number of times subjected to thermal hysteresis history by a heating treatment is great, the electrodes to be inspected are often formed in a state misregistered from the prescribed arrangement positions. When measurement of an electric resistance is performed as to the circuit boards to be inspected, which are large in area and have a great number of electrodes to be inspected formed in a state misregistered from the prescribed arrangement positions, by the electric resistance-measuring apparatus of the construction (i) or (ii), it is extremely difficult to electrically connect both electrodes for current supply and electrodes for voltage measurement at the same time to the respective electrodes to be inspected.
Description is given by a specific example. When an electric resistance as to an electrode T to be inspected having a diameter L of 300 μm as shown in FIG. 12 is measured, a clearance D between an electrode A for current supply and an electrode V for voltage measurement to be electrically connected to the electrode T to be inspected is about 150 μm. When the position of the electrode T to be inspected to the electrode A for current supply and the electrode V for voltage measurement deviates from the prescribed position shown in FIG. 12 by 75 μm in a direction that the electrode A for current supply and the electrode V for voltage measurement are arranged in alignment of a circuit board to be inspected as shown in FIGS. 13(a) and 13(b), electrical connection between either one of the electrode A for current supply or the electrode V for voltage measurement and the electrode T to be inspected is not achieved, and so necessary measurement of the electric resistance cannot be conducted.
As a means for solving such a problem, it is considered to make the clearance D between the electrode A for current supply and the electrode V for voltage measurement smaller, for example, to an extent of 100 μm or smaller. It is however extremely difficult in fact to produce such an electric resistance-measuring apparatus.
On the other hand, according to the electric resistance-measuring apparatus (iii), there is no need to form electrodes for current supply and electrodes for voltage measurement corresponding to individual electrodes to be inspected, and so capacity for misregistration to electrodes to be inspected becomes high even when a circuit board to be inspected which is subjected to the measurement of electric resistance is large in area and has a great number of electrodes to be inspected, and the small-sized electrodes to be inspected are arranged at a high density. In addition, such an electric resistance-measuring apparatus is easily produced.
However, such an electric resistance-measuring apparatus is great in measurement error range because it is a measuring apparatus according to the so-called pseudo-four-probe method, and so it is difficult to perform electric resistance measurement as to a circuit board low in electric resistance between electrodes with high precision.